This invention relates to the controlled thermal destruction of waste material, and, more particularly, to a method and apparatus for the treatment of hazardous and non-hazardous materials contained in a waste stream composed of organic (carbonaceous) and inorganic materials, such that the processed waste contains no residues that would require subsequent treatment or landfill disposal. The process involves high temperature pyrolysis and controlled gasification of organic materials, and metals recovery and/or vitrification of inorganic materials.
Placing medical and other waste in landfills was until recently the accepted method of disposal. When the consequences of landfill disposal were investigated more closely, public opposition and regulatory pressures restricted the landfill practice and forced the industry to instead employ incineration, the only other technology that was economical and seemed to solve the disposal problem. Incineration proved useful where landfill space was unavailable or too expensive, but it also was subjected to public concern. Eventually, new and more stringent air emission regulations became effective which most incinerators could not meet. Furthermore, those incinerators that have remained in operation leave an ash residue that often fails the E.P.A. toxicity characteristic leaching procedure (TCLP) test, and contains recognizable and potentially hazardous unburned materials.
Most existing medical waste incinerators are based on old technologies and were built during the period of 1960-1990, or before the present stricter regulations became effective. The air pollution control systems on the older designs, as well as the design of the incinerators, are inadequate to meet the present standards.
Although incinerators are somewhat effective for reducing the volume of waste by combustion, the basic nature of medical waste creates substantial problems for them. One of the major problems encountered in using incinerators to combust medical wastes is the heterogeneity of the waste material. This problem prevents the incinerators from maintaining a sufficiently high constant temperature to completely destroy all of the organic material in the waste. Pollutants that plague incinerators include the so-called products of incomplete combustion. For example, a first bag of such waste may be filled with containers of fluids, blood soaked bandages, and sharps (syringes, glass, metal surgical tools and the like), while a second bag may contain mostly plastics, paper, packing material, pads, surgical gowns, rubbers gloves, and the like. These two bags, fed independently into an incinerator, would create totally different combustion conditions. The first bag would quench and cool the combustion process, while the second bag would accelerate and raise temperatures.
During the low temperature cycle, products of incomplete combustion and the reformation of potentially hazardous organic materials, such as dioxin and furan, are much more likely. During the high temperature cycle, particulate, nitrogen oxide and metal oxide emissions increase, and particularly hexavalent chromium, a carcinogen. Shredding waste before feeding it into the combustion vessel would homogenize and mix the waste, but it is generally not acceptable because of the potentially infectious nature of the waste and the inherent problem of disinfecting a shredder having numerous internal components and small confined places where infectious material might collect and escape disinfection. Moreover, many states have laws prohibiting opening bags of infectious waste prior to their final processing.
Compounding the problem of temperature control within incinerators is the batch method of feeding that is commonly used. In this method, a ram system is normally used to push a charge of waste into a combustion chamber. Because the incinerator relies on the waste itself for fuel, as the waste combusts, chamber temperatures vary as the amount of combustible waste in the chamber changes. This problem is especially pronounced at start-up and shut-down. Temperatures also vary with changing feed rates and incinerators operate poorly at reduced feed rates.
It is important to achieve high temperatures because the destruction of inorganic waste components commonly found in medical and other waste streams requires them. Only a few incinerator designs can even reach the high temperatures required to melt stainless steel and borosilicate glass used in laboratories, and these incinerators further require fossil fuel additions to supplement the combustion process.
The destruction of organic waste also requires high temperatures, but instead of simply melting at high temperatures, such waste decomposes and burns if sufficient air is present. The combustion process can be self-sustaining only if enough heat energy is released during the process to cause additional material to decompose. This can be a problem in incinerators, however, and especially when wet and inorganic materials are present in the feed. Under such conditions, it is not possible to maintain a high, continuous operating temperature.
Apparatuses that have used plasma torches to improve on the low and varying temperature problem, have only achieved a partial solution. For example, U.S. Pat. No. 5,280,757 to Carter discloses a ram (or batch) feed system, which causes significant variation in gas flow rates and temperatures, and includes no precautionary measures to hold the exit gas temperature at a safe high level at which reformation of more complex organic compounds is minimal. The off-gas piping of Carter is composed of stainless steel and it leads to a steel cyclone for particulate collection.
The present inventors have determined that cooling gas in a six-foot section of stainless steel pipe causes the gas temperature to drop into a sufficiently low range (i.e., into the approximately 350.degree.-500.degree. C. range) to allow significant reformation of organic compounds, and particularly polycyclic aromatic hydrocarbons (PAH's). Carter discloses the presence of significant levels of PAH's in the emissions data in Table I. The present inventors have determined, then, that even when the gas temperature is sufficiently high and constant in the processing chamber to effect complete dissociation of organic materials, the gas must be maintained at a sufficiently high temperature after it leaves the chamber and until it is rapidly cooled, to reduce the likelihood of organic compounds reforming.
In recent years, public and regulatory attention has focused on the problems associated with the disposal of medical waste generated by hospitals, clinics, medical offices and research facilities. Numerous new technologies have emerged which been offered as solutions to these problems. Most of these new technologies employ disinfection and/or sterilization methods to reduce or eliminate the infectious portion of the waste so that it may be placed in a landfill after treatment. Where landfill space is still affordable, rendering the infectious component harmless, and then disposing of the entire waste stream as non-infectious waste in a landfill has been offered as a partial solution. As the amount of available landfill continues to decrease, however, and public opposition to using landfills increases, an alternative, final, no-landfill solution becomes inevitable.
Furthermore, all of the disinfection and sterilization technologies leave large amounts of residue or waste to deal with after the process is completed.
Also, many waste treatment processes require a strict sorting practice before selected waste items can be treated. Because most of the non-incineration technologies treat only the infectious portion of the hospital's waste, human error inherent in sorting the waste exposes the hospital to liability claims if infectious waste is later discovered in the non-infectious waste destined for landfills. Substantial fines and potential incarceration for repeat offenders in many states has made this a serious problem.